How to use <Reserved> correctly?

When changing artists in batches, can I add text before , such as Qiansheng Music , but if I add it like this, it will overwrite the original, which is not what I want

For specific questions, please check the attachment, thank you

If you use the tag panel then you can input only real characters but no variables.
The <keep> refers to "keep all the data that is already there as is". Any modification would adopt only that. (Which is more or less what you found out).

To append a string to a field, see the FAQs on How to append a string to a field

Can you tell me how to do it, thanks

Thank you, I just studied it myself and it succeeded. thank

Select the files that should get the same string appended.
Select the function Converter>tag-Tag
(as an example) Select the field ARTIST
Enter as
Format string: %artist% my appended text
Click OK
This adds the text "my appended text" to the already present data in the field ARTIST